The breakdown voltage and the operating resistance (On resistance or Rdson) are important characteristics of a power semiconductor device. The Rdson and the breakdown voltage of a power semiconductor device are inversely proportional. That is, the improvement in one adversely affects the other. To overcome this problem, U.S. Pat. No. 5,998,833 ('833 patent) proposes a trench type power semiconductor in which buried electrodes are disposed within the same trench as the gate electrodes in order to deplete the common conduction region under reverse voltage conditions, whereby the breakdown voltage of the device is improved. As a result, the resistivity of the common conduction region can be improved without an adverse affect on the breakdown voltage.
The buried electrodes shown in the '833 patent are electrically connected to the source contact of the device remotely, which may limit the switching speed of the device. Furthermore, the device shown therein may require at least one additional masking step.
U.S. Pat. No. 6,649,975 ('975 patent) and U.S. Pat. No. 6,710,403 ('403 patent) both show power semiconductor devices which include trenches that are deeper than the gate trenches to support field electrodes that are electrically connected to the source contact. The devices illustrated by the '975 patent and the '403 both require additional masking steps for defining trenches that receive field electrodes, which may increase the cost of production. In addition, the extra trenches increase the cell pitch and thus reduce cell density, which is undesirable.
U.S. patent application Ser. No. 11/110,467, assigned to the assignee of the present application, discloses a power MOSFET that includes an insulated source field electrode and two insulated gate electrodes in a common trench, in which a source contact makes electrical contact to the top surface of the source field electrode. As the cell pitch of such devices is reduced to obtain more current carrying capability per unit area, the width of the trenches and thus the width of the source field electrode decreases. As a result, the resistance of the source contact to the source field electrode increases.
It is, therefore, desirable to have an arrangement which may allow for the reduction of contact resistivity between the source contact and the source field electrode.